This removes the unsane "invalid_value" in favor of a clearer "hide_always"
option which also hides field on custom query page now.
Fixes small errors in post_process_request methode of the
DynamicFieldsModule component too and stricter follows PEP8,
especially when writing logic 'and'/'or' before a line-break.

_Beware:_ This version is non-functional yet and need more changes,
so please stick to previous versions, if you're not developing yourself.